The present invention relates generally to the field of medical electronic devices, and more particularly, to the field of implantable heart pacers which are substantially encapsulated in epoxy.
Generally, heart pacers are comprised of electronic circuit means for producing a suitable heart muscle stimulating output pulse and output means for conducting the said output pulse from the electronic circuit means to said heart muscle. Normally, an electrode is connected at one end into the particular heart muscle to be stimulated and at the other to a pacer which has been implanted or imbedded subcutaneously in the patient's chest, abdomen or other suitable location. With implantation of the heart pacer subcutaneously come various secondary problems which should be overcome in designing a safe and reliable heart pacer. Of particular importance is the insulation of a heart pacer firstly to protect the patient from internal contamination of the body from the materials used in the heart pacer, and secondly to protect the materials used in the heart pacer from detrimental corrosive attack by the body fluids which are prevalent in the implantation area.
In the past, heart pacers having been produced which encapsulate miniature circuitry within an envelope of epoxy. Other materials, such as plastics, silicon rubber and TEFLON(registered Trademark), have also been used for isolating purposes. Generally, these epoxy envelopes comprise rounded packets having formed therein an inert grounding plate for making electrical contact with the body and a negative output terminal for contacting the output electrode which is embedded in the heart muscle to be stimulated. Otherwise, the opoxy forms a smooth continuous insulating barrier between the body fluids which surround the heart pacer and the energy source and circuitry which together comprise the electronic circuit means for producing the desired rhythmic heart muscle stimulating negative output pulse.
Unfortunately, while providing a reasonable insulation of the body from the electronic circuit means of the heart pacer, encapsulation of a heart pacer with epoxy has not proven to be sufficient in insulating the various components of the heart pacer from the corrosive attack of the surrounding body fluids. Although there are, of course, a multiplicity of chemical constituents in the body fluids surrounding the implanted pacer, for the purposes of this discussion it sufficies to consider these fluids as being comprised mainly of an aqueous sodium and potassium chloride salt solution. In the past, a certain amount of difficulty has been encountered in producing pacers due to the phenomenon which is experienced when an epoxy encapsulated pacer is immersed for extended periods of time in such a solution. In particular, it has been found that the relatively small water molecules in that solution tend to penetrate the epoxy and to come in contact with the wires and other electrical components encapsulated therein. Even though the maximum voltage normally present within the pacer is normally within the range of 4-8 volts, and almost always less than 15 volts, this voltage is nonetheless sufficient to cause the electrolysis of water molecules contacting charged conductors. When positive and negative electrode components are positioned within a space containing an electrolyte, oxygen gas is formed at the positive electrode and hydrogen gas is formed at the negative electrode. Since, in the event of electrolysis, this gas is formed in a relatively confined area around the charged conductors and within the capsule, tremendous internal pressures could be created during the operation of the pacer, which pressures would have to be vented. This condition can be even more serious with the use of large surface integrated circuits in cans. Further, the larger the components, the greater the chance of breaking or cracking of the epoxy due to differences in expansion coefficients.
In prior art designs, batteries were often encapsulated within the epoxy capsule in close proximity to the electronic circuitry which produced the rhythmic pacer output pulse. Although the disposition of the energy source next to the electronic circuitry tended to reduce the length of the leads which were required to connect the energy source to the logic circuitry in the pacer, this juxtaposition tended to increase the likelihood that leakage of battery electrolytes from the energy source would damage or destroy the logic circuitry and/or the function of the device. Ions which cause electrolysis and electrochemical corrosion can also originate from leaky batteries, as for example, the normally used non-hermetically sealed mercury batteries. When it is remembered that pacers are normally implanted for extended periods during which their operation must be virtually infallible, it may be seen that any instances of malfunction which are caused by electrolyte leakage are not tolerable. Therefore, in order to overcome the problem that electrolyte leaking from a battery disposed near the logic circuitry of a pacer would be conducted along the conduit leading from the battery to the logic circuitry, various means have been employed in order to minimize the possibility that electrolyte will be transmitted to the interfere with the operation of the pacer's logic circuitry. In particular, leads have been employed which have a generally spiral or helical configuration which are intended to prevent the transmittal of electrolyte from the battery to the logic circuitry.